The present invention relates to machine vision systems and measurement of articles in quality control. In particular, the invention relates to the measurement of surfaces that are difficult to measure, including mirror and glass surfaces. Surfaces of this kind include automotive glasses that reflect light very poorly.
Conventional photogrammetric machine vision systems measure an object being viewed by focusing on them light spots or stripes and by taking images of the object using several cameras. The light pattern to be illuminated forms the object to be measured. The illumination apparatus can also consist of a number of illuminators or of a projector that reflects stripe patterns, enabling one to illuminate and measure several target spots simultaneously. The number of spots being illuminated can be increased by providing more illuminators, but this adds to the physical size of the illumination apparatus. An illumination apparatus alone will not often suffice to illuminate an entire article, instead the lights of an illuminator are often projected onto the desired location by projecting by means of a mirror. A number of cameras are used to take images of the measurement object illuminated by an illumination apparatus; and the locations of the spots are calculated using a central processing unit connected to the system.
The method referred to above is well suited for materials that reflect light. Instead, the method is poorly suited, for example, for the measurement of glass and plastic surfaces, particularly automotive glasses, which are bent in shape, because a regular laser light is not reflected from a glass surface in a manner allowing it to be imaged using cameras. In the case of automotive glasses, the measurement is particularly important also because of the fact that even small manufacturing defects can prevent installation of the glass in a car. Due to the importance of the problem, several different solutions have been developed to solve it.
In one solution, the piece of glass being measured is painted with a paint that reflects light better. Thereafter, the article can be measured using conventional photogrammetric machine vision systems. This method has the disadvantage of being expensive because prior to painting, the glass shall be cooled and washed right after the painting. Using an additional washing machine makes the production line more complicated and more expensive at the same time. In a version that has been further developed from this one, instead of paint, steam can be used that is condensed on the glass surface and need not be washed away. A disadvantage of this method is that the glass must be cooled so as to be so cold that steam is condensed on the glass surface.
In addition to conventional machine vision systems, specific machine vision systems for measuring glass have been developed. As in the manufacture of glass, a little of tin is left on the glass surface, which is due to the manufacturing method of glass, glass can be illuminated using an apparatus whose fluorescence in tin generates a visible measurement object. One such system has been described in publication U.S. Pat. No. 5,680,217, which discloses a method and system for measuring a bent glass sheet. In the method as shown in the publication, glass is imaged using an ultra violet laser with high intensity, allowing a sufficient amount of light to be reflected back from the glass to image it using, in other respects, however, conventional machine vision systems. The high price of an ultraviolet laser may also become a disadvantage of the solution presented. Furthermore, the high intensity to be used is risky, so due to work safety, the measurement arrangements must be implemented in an environment that is protected from light, because the intensities to be used can be harmful, for instance, to eyes. High intensities also involve problems with the camera devices to be used.
Conventionally, to measure the shape of glasses, mechanical sensors are used that are installed e.g. in a rack that supports the object being measured. In the case of an automotive windscreen, the windscreen is supported on the measurement rack by the same spots by which it would be fastened to the car. Thereafter, the measurement sensors are placed either beneath the glass or on top of it. Mechanical measuring instruments are complicated and slow. Being mechanical devices, they also are parts that wear and expose the glass being measured to measurement forces that can reduce the measurement accuracy. Further, a disadvantage of mechanical measuring instruments is that they are glass-specific, because each glass type must have their own measurement sensors in glass-specific measurement racks. One possibility to measure an article that is difficult to measure, that is to say a windscreen or a glass surface, is to measure and calculate the total reflection. This is, however, difficult because the objects being measured are of different shapes, making it nearly impossible to arrange the necessary total reflections.
Due to the above facts, there is an obvious need for a method and system that can be used to measure surfaces that are difficult to measure, such as glass, advantageously and efficiently.